Field of the Invention
The present invention relates to apparatus used to compensate for relative motion between a load lifting device and the load being lifted by the device. This invention, for example, can be used in an offshore crane that loads or unloads material from a floating vessel moored nearby, where the vessel is subjected to movement caused by waves.
When the sea is rough, a floating vessel may heave more than ten feet about a reference position. Unloading or loading must often be suspended because of the hazard to personnel and equipment. The relative motion between, for example, the hook connected to a winch in a crane mounted on an offshore structure and the deck of the vessel moored adjacent the structure, not only makes it difficult to attach a load to the hook, but also makes it dangerous to raise or to lower the load. For example, if a load of drill pipes or casing are attached to the hook and the lifting line is tensioned when the deck of the vessel is falling, the deck may subsequently rise faster then the rate at which the load is raised and the vessel would then collide with the load probably causing drilling pipes and casing to fall back onto the vessel. Moreover, once the hook has been attached to the load the vessel may fall beyond a point where the slack in the line is taken up thereby causing a sudden shock loading to be applied to the crane. Such a shock loading is deleterious to the crane and increases the hazard of load swing and hence collision between the load and the vessel or even personnel.
The apparatus of the present invention seeks to reduce, or to eliminate the latter problems whereby, for example, a supply vessel moored adjacent an offshore structure can be unloaded, or loaded in conditions during which work is normally suspended. This saves time and considerable expense in offshore drilling because work is normally suspended in foul weather when supplies cannot be loaded onto a rig.
The invention provides apparatus for use either in raising a load in a condition of relative motion between the apparatus and a support for the load, or in lowering a load in a condition of relative motion between the apparatus and a support for receiving the load. The apparatus comprises primary drive means for withdrawing, or for advancing a load supporting member such as line means, relative to the location of the load, as is also disclosed in U.S. Pat. No. 4,025,055, and tensioning means to apply and to maintain a tension in said load supporting member to compensate for said relative motion when the load is positioned on said support and is attached to said load supporting member.
The tensioning means can be arranged between the primary drive means and the load supporting member. For example, when the load supporting member comprises a line, a section of the line can be entrained about a pulley mounted on a traveling weight forming the tensioning means which is positioned between the primary drive winch and the load lifting block. The tensioning means is passively applied, responding only to line tension and gravity, not requiring external active control or power. Use of a suspended weight introduces a "zero-stiffness" element in the lifting means, which renders the lifting means totally compliant to motions of the load prior to lifting. Preferably, the tensioning means also comprise means to apply different tensions to the load supporting member, such as by adding or removing weight from the tensioning means.
A selectable unidirectional locking device such as a ratchet and pawl mechanism is provided to cause the tensioning means to become inactive in one direction of motion. This immobilizes the "zero-stiffness" element or traveling weight in the lifting system and allows the actual lift to proceed in a conventional fashion. The advantage of a ratchet and pawl mechanism is that the pawl can, if selected, travel freely over the teeth of the ratchet when a load is displaced in one direction (for example, when the deck of a supply vessel is rising) and it will then engage the teeth when the load moves in opposite direction (for example, when the deck of the vessel falls). This insures that the primary drive is effective at an optimium point during the relative motion (for example, at or near the crest of the vessel's motion). A further advantage is that the pawl is automatically disengaged if the load is again supported (for example, if the deck of the vessel meets the load when the deck rises faster than the load is being raised).
Prior art tensioning means comprise active control systems which operate on a "low-compliance" lifting system without modifying the inherent stiffness of that system. As such, they require a reliable source of substantial hydraulic power for their operation, since the disclosed devices usually include a piston and cylinder, or a rotary hydraulic drive coupled to a drum which stores a length of line. Failure of these tensioning devices would, at a minimum, make vessel loading or unloading impossible during adverse sea state conditions. But the major Problem of these hydraulically-actuated tensioning means is their requirement for applying a pretensioning load over a wide range of rapidly changing line velocities. In "low-compliance" stiff systems, this requires precise positioning, rapid control response and considerable power. In addition, the failure of any part of these complex systems during loading or unloading operations could cause loss of control over the load.
A device is contemplated which would prevent actuation of the tensioning means' ratchet and pawl device when the load is on the vessel's deck and the vessel is falling into the trough of a wave. If the operator activates the ratchet and pawl device as the vessel falls in the trough of a wave, the load will become suspended over the deck of the vessel. When the vessel subsequently rises, the load and vessel deck may impact. A device therefore need be employed which eliminates the possibility of vessel and load damage caused by operator misjudgment during operation of the tensioning means' unidirectional locking device.